ΔE 11.8, fair: the same colour family, but not a straight swap. Niblet Green is a little lighter, more saturated, cooler than Resedagrün (RAL 6011).
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Niblet Green is a little lighter, more saturated, cooler than Resedagrün (RAL 6011).
The same difference applies the other way round: Resedagrün (RAL 6011) is a little darker, more muted, warmer.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Resedagrün (RAL 6011) on the colour wheel See Niblet Green on the colour wheel
All matches: Resedagrün (RAL 6011) equivalents · Niblet Green equivalents